Continuous light enhances adventitious rooting in Robinia pseudoacacia L. through a regulatory module involving RpCOP1 and auxin signaling

Continuous light is a crucial environmental signal, yet its precise role in adventitious rooting in woody plants, particularly concerning the function of the classical light-signaling repressor CONSTITUTIVE PHOTOMORPHOGENIC 1, remains a significant open question. This study investigated how continuous light regulates adventitious root formation in Robinia pseudoacacia L. (black locust) through RpCOP1-mediated mechanisms. Black locust tissue culture plantlets were analyzed through anatomical observation, physiological measurements, hormone profiling, transcriptome sequencing with time-ordered gene co-expression network analysis, and heterologous overexpression validation in poplar. Continuous light treatment advanced rooting by 2 days, increasing adventitious root number by 86.03 % and root length by 110.76 %. The pro-adventitious rooting effect was associated with physiological reprogramming, wherein continuous light reduced photosynthetic efficiency while promoting soluble sugar accumulation at the stem base. Time-ordered gene co-expression network analysis identified RpCOP1 as a key upstream hub whose expression is induced by continuous light. Upregulation of RpCOP1 coincided with systemic hormonal reprogramming, highlighted by 60.72 % increase in auxin. Functional validation via heterologous overexpression of RpCOP1 in poplar supported this positive regulatory role. Compared with wild-type, transgenic lines under continuous light exhibited 33.33 % increase in adventitious root number and 75.85 % increase in root length, demonstrating a photoperiod-dependent effect, with rooting performance ordering continuous light > normal light > continuous dark. These findings support a context-dependent model where RpCOP1 function exhibits either repressive or activating roles depending on light environment and tissue type.